Today, electronic mail (also commonly referred to as email) provides great convenience in communication and thus, is widely used for many purposes, including both personal and business purposes. Unfortunately, some entities, such as unscrupulous marketers, pranksters, etc. often abuse the use of electronic mail by sending out thousands of unsolicited electronic mail messages to others. Such unsolicited electronic mail messages are commonly referred to as spam and senders of spam are commonly referred to as spammers. Besides annoying the recipients, such unsolicited bulk electronic mail often takes up significant storage space in the recipients' electronic mail account and slows down network traffic.
To tackle the problem of unsolicited bulk electronic mail, some conventional spam filters have been developed to screen out unsolicited bulk electronic mail by some predetermined texts, such as words, phrases, sentences, etc., in the body of the electronic mail. Specifically, some users may have reported some unsolicited bulk electronic mail, which is analyzed to identify the words, phrases, sentences, etc. commonly used in the content of the unsolicited bulk electronic mail.
However, many spammers circumvent the above conventional text-based spam filters in various ways, such as by inserting texts commonly used in legitimate emails, deliberately misspelling some words, etc. Since spammers may not be able express their messages in text without being detected by these conventional text-based spam filters, some spammers create graphic images containing the spammers' messages and insert such graphic images into the body of spam. Such graphic images are usually in a format that can be rendered by client machines commonly used by the recipients, such as Joint Photographic Experts Group (JPEG) format, graphics interchange format (GIF), etc. Since the conventional text-based spam filters only check the text in electronic mail, the graphic images embedded in electronic mail are not checked. Therefore, unsolicited bulk electronic mail containing graphic images may pass through the conventional text-based spam filter without being detected.